So, with that in mind, please read the following from Curtis over at Blue in Red Zion, with commentary by me:
1) Always use formal titles - Always refer to the person you are writing to with their formal title, and do it throughout the message. Even if you know the person personally, I would still strongly recommend calling them Representative or Senator.
Last year, when I was on the Hill, I made the mistake of calling my favorite English Teacher I never had at Olympus by the wrong name. I called he Carol, which earned me a glare. The I called her "Ms. Moss, I mean Spackman-Moss," which got me another glare, until I came up with the appropriate name: "Representative Spackman-Moss."
2) Be sure to reference the specific bill you are interested in by bill number and name. The proper format is H.B. (for "house bill" as an example) 1 - the name of the bill here. If you are writing someone other than the bills sponsor, also be sure to include the name of the person proposing the bill.
I've heard from many people who have either been Legislators or interns that a Legislator will read several letters/emails about the same bill at once, so this makes it easier to get your letter read when it will do the most good. Also, with as mny bills as there are every year, it's hard to keep up with them and will allow them to look it up.
3) Always mention why your opinion matters - more specifically, state that you live in the legislators district, city, county, or are a citizen of Utah. If you involved with an organization relating to an issue, mention that as well - if this is the case be sure to point out that you are acting as an individual.
Also, try to only contact your representative and/or the bill's sponsor. This is the most effective use of time.
4) Keep messages short, generally less than three or four paragraphs.
Because do you really want to read a novel in your inbox. (I know that because of it's length, 50% of my readers won't read this far.)
5) Avoid using arguments that are based on ideology - quoting scripture, party rhetoric, and/or sources that are less than reputable will make legislators ignore you and your message quick. This is especially true if you disagree with bill. If you agree, this still harms your argument and may cause a legislator to reevaluate their positions. Similarly, do use logic whenever possible; oftentimes when legislators write bills, they do not know about potential consequences to groups if the bill were to pass.
6) If possible, be sure to include personal stories that explain how this bill would help or hurt you, your family, your friends, your organizations, etc.
#6 is my favorite. If a bill riles you up enough to contact your legislature, then there is a reason for it. Tell them!
7) Be sure you have read the bill, or at the very least, read a summation of the bill from a reliable source, before writing - if you make inferences and logical leaps that are just not there, the legislator may ignore future correspondence from you.
I'm amazed at people who complain about something (a bill, an article, a blog post) that suggests something that will make it better. Something that is already in there. (.e. "Your post would have been better if you had made additional comments.)
8) Always sign your name to any message, it adds legitimacy and show you believe in your cause.
It makes you 50% less crazy.
And, I'd like to add a #9:
Write, even if you think your legislator agrees with you. A few years ago, there was an issue that I cared about being discussed at the Legislature, which I though my Representative was in agreement with me on, so I decided not to waste her time. When she voted the other way, she sent a reply that said that she had heard from more constituents disagreeing with her after her vote than had weighed in on either side before the vote. She honestly didn't know which way to go, and went with what she though best, a position that changed once constituents weighed in on the issue.